Electrostatic spray processes deposit coating materials with great efficiency, thus minimizing waste and pollution, but they normally demand that the base to be coated be conductive so that the charge can be carried away to a ground. This is easily carried out when the substrate is conductive, but dried wood and plastics are insulators, so the charge builds up on the substrate being coated to repel the charged coating particles. This destroys the effectiveness of the electrocoating process.
It is known to pretreat the surface of the insulator to render it conductive, and aqueous and nonaqueous solutions containing ionizable compounds, both organic and inorganic, have been used for this purpose. As a result, electrostatic spray processes are now commonly used to coat wood and plastic surfaces.
It is desired to maximize the effectiveness of the electrostatic coating of insulators, and especially wood, where a plurality of coatings must be applied in order to provide the desired finish. The difficulty is that the coatings are of insulating character, and it is desired that the initial pretreatment of the substrate continue to provide a capacity to receive charged spray particles after several coatings have been applied.
To achieve this result, it is known to expose the pretreated and coated objects to high humidity in order to enhance the conductivity of the pretreatment, but even this expedient fails after a few coatings. Unfortunately, the normal finishing of wood for furniture use involves many coatings, so superior pretreatments are needed to extend the number of coatings which can be eftively received by the pretreated substrate (following humidification in known fashion).
It is also known that conductive pretreatments adversely effect wood. Thus, and while aqueous pretreatments are broadly useful herein, they can injure wood and solvent solutions are preferred. Agents which damage or discolor the wood (either immediately or with time) or which interfere with adhesion of the coatings to one another or with the capacity of the coating and the substrate to resist changing environments, are preferably avoided.
Also, the conductive agents are sensitive to moisture, and if these are present in the finished coating, they can render it sensitive to moisture. This causes the coating to blush (whiten) in high humidity atmosphere at room or lower temperature. As will be evident, this adverse result must also be avoided.